1. Field of the Invention:
This invention relates to a laser optical system for use in an image forming device for writing an image on a photosensitive recording medium by exposure of the recording medium to a plurality of laser beams of different wavelengths.
2. Description of the Prior Art:
Japanese Patent Laid-Open SHO No. 60(1985)-201,319 discloses a laser optical system for incorporation in an electrophotographic color printer. This printer comprises a printer proper provided in the interior thereof with a photoconductive unit and a laser source unit. This unit incorporates therein three semiconductor lasers which are modulated in accordance with the input information and are caused to emit laser beams of mutually different wavelengths. Collimating lenses are disposed in front of the semiconductor lasers for producing parallel beams of a prescribed diameter from the laser beams. The laser beams which have passed through the lenses are scanned with a polygon mirror, and then caused to impinge on the photoconductor. As the result, an image conforming with electric signals indicative of the aforementioned input information is written in an electrophotographic recording medium.
U.S. Pat. No. 4,728,965 discloses a laser printer which is provided with three separate channels for printing a color image on a receiving medium such as a photosensitive film. These channels are severally provided with gas lasers for emitting lasers of specific wavelengths. A beam combined receives the light beams from the three channels and combines them to form a combined beam spot. The combined beam spot is relayed to a polygon mirror which scans the beam onto a receiving medium or a recording medium.
A laser optical system, which has been considered by the inventor, in a laser printer possessing a plurality of semiconductor lasers for the purpose of writing an image conforming with electric signals indicative of input information is illustrated in schematic form in FIG. 5.
The laser printer illustrated comprises three semiconductor lasers 11, 12, and 13, which are modulated in accordance with printed information by a controller (not shown) and caused to emit laser beams 11a, 12a, and 13a of mutually different wavelengths. In front of these semiconductor lasers 11 to 13 are disposed collimating lenses 14 to 16 which produce a bundle of parallel beams from these laser beams. The laser beam emitted from the semiconductor laser 11 and relayed by the collimating lens 14 is reflected by a reflecting prism 17 and is superposed in a synthetic dichroic prism 18 on the laser beam from the semiconductor laser 12. On the superposed laser beams, the laser beam from the semiconductor laser 13 is further superposed similarly in the synthetic dichroic prism. The three laser beams thus superposed are then caused to impinge on a polygon mirror 20 having a multiplicity of facet mirrors formed on the peripheral surface, to be scanned thereon. The scanned beam is caused to impinge on a photosensitive recording medium 24 such as a photosensitive film by means of a f.theta. lens 21, a folding mirror 22, and a window 23. This photosensitive film 25 is conveyed between an imaging drum 25 and two conveyor rollers 26 pressed against the drum 25 and forwarded to a developing device (not shown). To sense the position for starting the scanning operation, a mirror 27 is interposed between the f.theta. lens 21 and the folding mirror 22. The beam of light reflected by the mirror 27 is detected by a receiving sensor 28.
In the laser optical system illustrated, since three collimating lenses 14 to 16 are disposed to serve as many semiconductor lasers 11 to 13, these collimating lenses 14 to 16 must be disposed at fixed positions relative to the respective semiconductor lasers 11 to 13 and adapted to swing within fixed angles. Further, the reflecting prism 17 and the two dichroic prisms 18, 19 which jointly serve the purpose of superposing the three laser beams must be disposed at fixed mutual positions and with fixed angles. During the production and the maintenance of the laser printer, therefore, the adjustment of the positions and the angles mentioned above calls for much time and labor. Particularly the mutual positional adjustment of the two prisms is an extremely difficult optical adjustment for superposing one straight line over another straight line. This optical adjustment coupled with the adjustment of collimating lenses is difficult to attain with the conventional technique.